1. Field of the Invention
The present invention relates to a method for determining the mass moment of inertia of an electric motor drive system of a machine, in particular a machine tool, which includes a drive motor and further drive elements arranged downstream of the drive motor.
2. Discussion of Related Art
For assessing the dynamic machine layout, the ratio of the mass moment of inertia of the motor (which is related in a strict sense to the drive motor, i.e. its rotor, and which, in connection with a rotary drive mechanism, is equal to the mass moment of inertia of the rotor) to the mass moment of inertia of the load (which relates to the remaining components of the electric motor drive, for example a clutch, a ballscrew, further gear elements, a table which can be moved by the drive mechanism, etc.) must be known. The mass moment of inertia of the motor and the mass moment of inertia of the load must have a defined relationship to each other in order to achieve a high quality of the control of the electric motor drive mechanism.
The quotient of mass moment of inertia of the motor and mass moment of inertia of the load is a definitive quantity for judging the control capability, as well as the control quality of an electric motor drive system. If the quotient lies outside the range wherein it allows a high-quality control, steps for further optimizing the control often do not bring the desired success.
Moreover, from the (total) mass moment of inertia of the drive system, i.e. the sum of the mass moment of inertia of the motor and the mass moment of inertia of the load, and taking into consideration the maximum current for the drive motor, it is also possible to calculate the maximally achievable acceleration. Customarily a value between 1 and 2 is desired for the quotient of the mass moment of inertia of the motor and the mass moment of inertia of the load. As a rule, this is already being taken into consideration in the course of producing a machine, for example a machine tool, by the manufacturer. However, subsequent changes in marginal values such as, for example, a later change of a component of the machine, lead to changes of the quotient, so that it can assume a value outside of the desired range. After changes in a machine have been made, this can in turn lead to having to use a different drive motor in order to make the control of the drive mechanism possible with sufficient quality.
Since in the strict sense the value of the mass moment of inertia of the drive motor is known as a rule and represents a constant quantity (provided no changes were made in the drive motor itself), it is sufficient to determine the mass moment of inertia of the electric motor drive mechanism as a whole. If the mass moment of inertia of the motor is subtracted therefrom, the result is the mass moment of inertia of the load, which in turn allows the calculation of the actual value of the quotient of the mass moment of inertia of the motor and the mass moment of inertia of the load.
A determination system for a control constant of an electric motor used for driving a machine tool is described in EP 0 827 265 B1 for determining control constants by which it is possible, inter alia, to determine the mass moment of inertia. In this case an iterative method is employed, which is based on a correction model and the inclusion of control deviations in the course of controlling the motor. However, an iterative method always has the disadvantage that it is limited with respect to its accuracy, and furthermore, because of its proneness to errors, it does not necessarily lead to an optimal result for all operational states. Note that EP 0 827 265 B1 corresponds to U.S. Pat. No. 6,037,736, the entire contents of which are incorporated herein by reference.